I would tend to agree that most people don't really need full frame digital, and might be better off with the greater convenience and lower cost of a smaller format. And maybe younger users will prefer EVFs, and maybe systems with mirrors and optical viewfinders will even one day become dinosaurs.

But why wouldn't some of these mirrorless cameras of the future be APS-C? Where is the great advantage for m4/3 there?

The Nikon APS-C sensor diagonal is 28.2mm. That's 15.0mm less than Nikon full-frame. The Olympus m4/3 sensor diagonal is only 6.6mm less than the Nikon APS-C. And yet he claims that "the APS-C sensor requires lenses that rival traditional DSLR lenses in size and weight."

It would be more accurate to say m4/3 requires lenses that rival APS-C in size and weight. Some may feel the m4/3 size is better, but it's not going to be by enough for m4/3 to really have any disctinct identity there. A 6.6mm smaller image circle (1/4 inch) is not going to be enough to declare Olympus and Panasonic the de-facto winner of any future battles for the hearts and minds of consumers in the compact interchangable lens segment.